The present disclosure relates to a nozzle box assembly and, more particularly, to a nozzle box assembly provided to the stage inlet of a steam turbine so as to inject steam to the stage.
A nozzle box assembly for a steam turbine, as shown in FIG. 1 and FIG. 2, includes three constituent elements, that is, a torus 14, a bridge ring 16 and a steam path ring 12. Each of the constituent elements is prepared as a 180° segment in the initial stage and then the constituent elements are welded in sequence so as to form two nozzle box halves 18. FIG. 1 and FIG. 2 show one of the nozzle box halves 18, wherein the other one also has the same shape and structure.
Next, the two halves 18 are joined together along a horizontal center line so as to form a nozzle box assembly for a steam turbine. Each of the nozzle box halves 18 includes one or more steam inlets 10, which are integrally formed with the torus 14. The steam inlets 10 are connected to the torus 14 on a plane surface, which is perpendicular to the rotation shaft of the turbine.
During the operation of the steam turbine, steam from a steam supply source such as a boiler and the like is introduced through the steam inlets 10 and flows in the torus 14. The flow direction of the steam is typically changed to the axial direction such that the steam flows through the annular opening of the bridge ring 16 to the inside of the steam path ring 12. The steam path ring 12 is provided with a series of nozzles, including airfoil vanes 13 for directing the steam flow.
The nozzle box assembly as described above has the configuration, in which the torus 14, the bridge ring 16 and the steam path ring 12 are coupled together. More specifically, as for this coupling, the bridge ring 16 and the steam path ring 12 are respectively formed in advance and then the steam path ring 12 is welded to the bridge ring 16 after the bridge ring 16 is welded to the torus 14, thereby achieving the mutual welding.
The coupling method as above employs a method for forming each of the bridge ring 16 and the steam path ring 12 integrally or in a semicircular body. In this case, if partially defective bridge rings 16 or vanes 13 are generated in the process of manufacture, the whole product has to be abandoned. Therefore, the coupling method is inefficient in terms of manufacture and excessive labor is required for the welding.
Further, if the bridge ring 16 or the vane 13 is partially damaged during the operation of the steam turbine, the welding coupling has to be released and then the whole bridge ring 16 or the steam path ring 12 has to be replaced, resulting in the difficulty of maintenance.